<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        setTimeout(function () {
            console.log('hah')
        }, 2000);
        console.log('hehe')
        /*
        同步 a -> b ->c
        异步 a   ->b
               -> c -> d 
        */
        let num = 0  
        setInterval(function(){
            num++
            console.log(num)
        }, 1000);
    </script>
</body>

</html>